tools/rmmod: rework control flow

tools/rmmod: rework control flow

Currently the control flow we use is a bit tricky:

 - we use a goto within a loop, to avoid one function call
 - we increment r, assuming it cannot wrap around

In practise these are not bugs, although make the code more convoluted
than needed. While in here, scope the local err to where it's used.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/180
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>